1. Field of the Invention
The present invention relates to photodiodes and, more particularly, to a photodiode that reduces the effects of the surface electron-hole recombination sites.
2. Description of the Related Art
A p-type material is a semiconductor material, such as single-crystal silicon, that includes impurity “acceptor” atoms (atoms that are missing one electron in their outer shell). All of the column IIIA elements (group 13) (Boron, Aluminum, Gallium, Indium, and Thalium) have one missing electron in the outer shell and are acceptor atoms.
FIG. 1 shows a graph that illustrates the dopant profiles of the column IIIA elements in silicon when implanted at the same dose at the same energy for the same amount of time. As shown in FIG. 1, as the size of the atom increases, the depth of the implant decreases. As a result, the element boron, which is the smallest atom, has the deepest implant.
FIG. 2 shows a graph that illustrates the dopant profiles of each of the implanted column IIIA elements when thermally cycled in a neutral ambient for the same amount of time. As shown in FIG. 2, after thermal cycling, the smallest atom, boron, diffuses the most, while the largest atom, thalium, diffuses the least, essentially none at all.
As further shown in FIG. 2, the element indium also diffuses very little due to the large size of the atom. Indium is not commonly used in semiconductor fabrication because indium has a maximum dopant concentration of approximately 5×1017 to 1×1018 atoms/cm3, which is an LDD dopant concentration. This is due to the solid solubility and the tendency of indium to precipitate out.
A photodiode is a semiconductor device that is commonly formed by combining a p-type material with an n-type material. When exposed to electromagnetic radiation, such as visible light, a photodiode generates a number of electron-hole pairs at different depths within the device depending upon the depth at which the electromagnetic radiation was absorbed by the device.
Black and white photodiodes are photodiodes that can collect photons over all or substantially all of the visible spectrum, which includes many colors, and therefore generate electron-hole pairs over a large vertical range within the photodiode. On the other hand, color photodiodes, such as blue, green, and red photodiodes, are photodiodes that collect photons over a much narrower range of wavelengths, and therefore generate electron-hole pairs over a much smaller vertical range within the photodiode.
Black and white photodiodes can be used as color photodiodes by placing a color filter, such as a blue, green, or red filter, over the black and white photodiode. Thus, although a black and white photodiode is capable of capturing a wider range of the visible light spectrum, the color filter limits the photodiode to collecting only a single color.
FIG. 3 shows a cross-sectional diagram that illustrates a prior-art blue photodiode 300. As shown in FIG. 3, blue photodiode 300 includes a first p-type (boron) region 310, such as a substrate, a second n-type region 312, such as an epitaxial layer or well, that overlies and contacts p-type region 310, and a depletion region 314 that is formed across the pn junction between p-type region 310 and n-type region 312.
Photodiode 300 can be formed with a blue filter and a large depletion region 314 that collects blue light, or can be formed without a blue filter and with a small depletion region 314 that collects substantially only blue light. Further, photodiode 300 includes an isolation region 316, such as oxide, that is formed on n-type region 312.
In operation, blue photodiode 300 is first reset by placing a reset voltage on n-type region 312 that reverse biases the pn junction. The reverse-biased voltage, which sets up an electric field across the junction, increases the width of depletion region 314 so that the blue portion of the electromagnetic spectrum can be absorbed in depletion region 314.
Once photodiode 300 is reset, photodiode 300 is then exposed to a source of electromagnetic radiation for an integration period. When photodiode 300 is struck by electromagnetic radiation during the integration period, the radiation penetrates into the semiconductor material down to an absorption depth that depends on the wavelength of the radiation.
For example, blue light has an absorption depth of approximately 0.7 microns, while red light has an absorption depth of approximately 1.2 microns (measured down from the top surface of n-type region 312 when n-type region 312 is formed as an epitaxial region, or measured down from the top surface of p-type region 310 when n-type region 312 is an implanted region in region 310).
Blue wavelengths of light are absorbed in depletion region 314 which, in response, generates a number of electron-hole pairs in depletion region 314. The electric field set up across the reverse-biased pn junction attracts the electrons that are formed in depletion region 314 (along with the electrons that are formed in p-type region 310 within a diffusion length of depletion region 314) to n-type region 312 where each additional electron reduces the magnitude of the reset voltage that was placed on n-type region 312.
Thus, at the end of the integration period, the total number of electrons collected by n-type region 312 has reduced the reset voltage to an integrated voltage. As a result, the total number of electrons collected by n-type region 312 during the integration period, which is a measure of the intensity of the blue electromagnetic radiation, can be determined by subtracting the integrated voltage from the reset voltage.
One problem with photodiode 300 is that photodiode 300 has a large number of electron-hole recombination sites that are located at the boundary between n-type region 312 and isolation layer 316. The surface electron-hole recombination sites, in turn, consume a number of the photo-generated electrons.
The lost electrons reduce the total number of collected electrons which erroneously increases the magnitude of the integrated voltage. As a result, the surface electron-hole recombination sites of photodiode 300 reduce the intensity of the blue signal that is calculated by subtracting the integrated voltage from the reset voltage.
Thus, there is a need for a photodiode that reduces the effects of the surface electron-hole recombination sites.